PHYSIOLOGY AND BIOCHEMISTRY OF SYMBIOTIC AND FREE-LIVING CHEMOAUTOTROPHIC SULFUR BACTERIA

In this chapter, the known mechanisms that enable diverse sulfur-oxidi zing chemoautotrophic bacteria to conserve energy are summarized. The mechanisms known to be utilized by symbionts constitute a relatively s mall subset of those used by free-living chemoautotrophic bacteria; th erefore, a search for additional pathways in symbionts should be fruit ful. The emerging evidence for the use of nitrate as an alternative el ectron acceptor in sulfur oxidation by two types of symbionts is also discussed. Thus far, the data are not completely consistent with the o peration of either a classical dissimilatory or classical assimilatory nitrate reductase. Lastly, previous literature calculations regarding the efficiency of coupling between sulfur compound oxidation and carb on dioxide fixation are reexamined. For both free-living and symbiotic sulfur bacteria, the published efficiencies are shown to have been ov erestimates by up to four-fold due to an inappropriate assumption abou t the source of electrons used for carbon reduction.